Conventionally, as a nonvolatile memory device, a NAND flash memory has been developed. The NAND flash memory is a device for storing data by injecting charge into a floating gate electrode made of silicon to change the threshold of a field effect transistor including this floating gate electrode. In such a memory device, increasing the memory density requires shrink of the structure. However, shrink of the structure causes the problem of degraded retention characteristics for charge injected into the floating gate electrode.